The present invention relates generally to the removal of hydrogen (H2) and carbon monoxide (CO) impurities from gas streams. In particular, the invention relates to the removal of these impurities from air. The invention has particular application in the production of high purity (HP) and ultra-high purity (UHP) nitrogen (N2) gas.
In many chemical processes, carbon monoxide and hydrogen are undesired species in nitrogen because of their chemical reactivity. For example, the electronics industry requires UHP nitrogen (typically, CO and H2 each less than 10 parts per billion by volume (“ppbv”)) to provide an inert atmosphere for the production of semiconductor materials. Impurities present in the nitrogen during formation of the silicon wafers greatly increase chip failure rates.
When air is subjected to cryogenic separation to produce nitrogen, carbon monoxide present in the feed air will essentially end up in the product nitrogen since carbon monoxide and nitrogen have similar boiling points. Hydrogen enriches in the product nitrogen to approximately two times its concentration in the feed air. Therefore, the production of UHP nitrogen, i.e. nitrogen that is at least substantially free of carbon monoxide and hydrogen, for the electronics industry requires processes for removing hydrogen and/or carbon monoxide from the air or nitrogen stream.
In the conventional process for cryogenic separation of air to recover nitrogen and oxygen (O2), feed air is compressed, cooled to low temperature, and then introduced to a cryogenic distillation unit (otherwise known as air separation unit or ASU), that usually involves one or two distillation columns. If not removed, water and carbon dioxide present in the feed air will freeze out and block heat exchangers employed for cooling the gas prior to distillation. The separation unit used to remove water and carbon dioxide is commonly referred to as the Front End Unit (FEU).
Before entering the FEU, atmospheric air is compressed to an elevated pressure from 50 to 150 psig (0.45 to 1.1 MPa), followed by water cooling and removal of condensed water. Throughout the document, pressures given in metric units are calculated on an absolute basis. The cooled air, which is then about 100° F. (38° C.), can be further cooled to 40° F. (4.5° C.) using combination of a water chiller tower and Direct Contact After Cooling (DCAC). The bulk of the water present in the air is removed by condensation and phase separation. The gas is then passed to a molecular sieve bed or mixed alumina/molecular sieve beds of the FEU where the remaining water and carbon dioxide are removed by adsorption. This separation takes advantage of the fact that both water and carbon dioxide are much more strongly attracted to the solid adsorbents than oxygen and nitrogen, so they are preferentially removed from the gas stream by the adsorbent. The air stream exiting the bed, which is at least substantially free of carbon dioxide and water, is then sent to the cryogenic distillation unit.
Eventually, the capacity of the adsorbent to adsorb water and carbon dioxide is exhausted and water and/or carbon dioxide will begin to “break through” the adsorbent bed and leave the FEU. The exhausted bed is taken “off-line”, i.e. off feed gas, and regenerated to desorb some of the water and carbon dioxide, and restore the adsorption capacity of the adsorbent. To achieve constant feed and product gas flow rates, at least two adsorbent beds are used, one bed “on-line” operating under the adsorption step while the or each other bed is regenerated, their roles being periodically reversed in the operating cycle.
Bed regeneration is conducted by heating the bed to higher temperature (thermal swing adsorption, or TSA—see, for example, U.S. Pat. Nos. 4,541,851 and 5,137,548) or by decreasing the pressure of the gas in the bed with no heating (pressure swing adsorption, or PSA—see, for example, U.S. Pat. No. 5,232,474). The heating step of the TSA approach can be conducted at the original feed pressure, or more commonly, at a lower pressure of 2 to 15 psig (0.1 to 0.2 MPa). In any case, a flow of a gas that is at least substantially free of carbon dioxide and water is used to simultaneously purge the beds. The present invention involves a TSA regeneration step, so the case of PSA regeneration is not considered further.
During thermal regeneration, part of the purified air from the bed on feed, part of the UHP nitrogen product gas from the cryogenic distillation unit, or some of the waste stream from the cold box is heated to 200 to 250° C. The hot gas is passed through the adsorber bed being regenerated for a period of time equal to perhaps half of the total regeneration time. This step is then followed by flowing cool regeneration gas (e.g. at 5 to 30° C.) for the remainder of the regeneration time, thereby cooling the bed to that temperature. Regeneration is usually carried out in a countercurrent direction with respect to the adsorption step and is typically conducted at the lower pressure of 2 to 15 psig (0.1 to 0.2 MPa).
The conventional TSA FEU is quite capable of removing carbon dioxide and water from air. However, alumina or molecular sieve beds are not effective for the removal of carbon monoxide or hydrogen. Applications for UHP nitrogen in the electronics area often stipulate both hydrogen and carbon monoxide specifications. Thus, there is a need for processes for the combined removal of carbon monoxide and hydrogen from air.
Two approaches are typically considered to produce nitrogen that is at least substantially free of carbon monoxide and hydrogen. The first approach involves selective removal of carbon monoxide and hydrogen from the nitrogen product of the ASU (nitrogen post-treatment), and the second approach involves oxidation of carbon monoxide and hydrogen in the feed air followed by removal of carbon dioxide and water in the FEU (air pre-treatment).
In the area of nitrogen post-treatment, U.S. Pat. No. 4,713,224 teaches a one step process for purifying nitrogen containing trace quantities of carbon monoxide, carbon dioxide, oxygen, hydrogen and water in which the gas stream is passed over a material comprising elemental nickel and having a large surface area. Carbon monoxide, hydrogen, carbon dioxide and water are all chemisorbed or catalytically oxidized and subsequently removed. This approach is feasible only for nitrogen post-treatment, as the relatively high levels of oxygen in an air feed would oxidize the nickel and render it ineffective.
U.S. Pat. No. 4,579,723 discloses passing an inert gas stream containing trace levels of carbon monoxide, hydrogen, oxygen, and carbon dioxide through a catalyst bed containing a mixture of chromium and platinum on γ-alumina followed by a second bed composed of γ-alumina coated with a mixture of several metals. The first bed oxidizes carbon monoxide and hydrogen, and adsorbs water, while the second bed adsorbs carbon dioxide and oxygen, yielding a high purity product (less than 1 ppm impurities). These metallic catalysts are expected to be expensive.
Literature exists showing that carbon monoxide can be effectively adsorbed or chemisorbed on various adsorbents, and this suggests another approach for nitrogen post-treatment. U.S. Pat. No. 4,944,273 suggests that carbon monoxide can be selectively adsorbed by zeolites doped with metals such as calcium (Ca), cobalt (Co), nickel (Ni), iron (Fe), copper (Cu), silver (Ag), platinum (Pt), or ruthenium (Ru). U.S. Pat. No. 4,019,879 discloses the use of a zeolite containing copper (Cu+) ions for adsorbing carbon monoxide selectively. U.S. Pat. No. 4,019,880 describes the adsorption of carbon monoxide on zeolites containing silver cations. The carbon monoxide concentration can be reduced to levels as low as less than 10 ppm. U.S. Pat. No. 7,524,358 also describes use of silver-exchanged zeolites for carbon monoxide adsorption.
The more common approach for producing UHP nitrogen is air pre-treatment and involves oxidizing carbon monoxide and hydrogen in the feed gas and then removing carbon dioxide and water in the FEU. This approach is attractive because the oxidation reactions in the air stream are very favorable thermodynamically and equilibrium conversion is essentially complete. In addition, the by-products formed by the process are conveniently handled by the existing FEU.
The oxidation of carbon monoxide to carbon dioxide and of hydrogen to water in the presence of oxygen occurs readily in the absence of catalysts at high temperatures (e.g. above 500° C.). Oxidation at lower temperatures in moist air usually requires catalysts. Lamb and Vail (J. Am. Chem. Soc., 1925, 47 (1), 123-142) show that hopcalite catalyst can nearly completely oxidize carbon monoxide at 100° C., but the activity of the catalyst is diminished with increasing water content in the feed gas and the activity for hydrogen removal is essentially negligible at these same conditions. Hopcalite catalyst comprises a mixture of copper oxide and manganese oxide.
The carbon monoxide and hydrogen reactions can be carried out at modest temperature, e.g. at about 150° C., in the presence of precious metal catalysts based on palladium (Pd) or platinum (Pt) (Anderson, H. C. and Green, W. J., Ind. Eng. Chem., 53, 645, 1961). Thus, one approach for removing hydrogen and carbon monoxide from air is to pass compressed, heated air through a reactor vessel containing precious metal catalyst, then cool the effluent stream and reject water and carbon dioxide in the FEU (see, for example, U.S. Pat. No. 5,656,557). The main disadvantages of this removal technique include (i) the need to heat the air prior to introduction to the catalyst bed; (ii) the need for an extra heat exchanger and an extra booster heater that result in increased plot space; (iii) the need for more power to accommodate the additional system pressure drop and heat duty; and (iv) the relatively high capital cost associated with the reactor vessel, heaters, and expensive noble metal catalyst.
U.S. Pat. No. 6,074,621 mitigates some of these issues by utilizing the heat of compression of the feed air to provide warm air to a bed of noble metal catalyst for carbon monoxide oxidation, followed by cooling, water removal in the FEU, hydrogen removal by oxidation in a layer of noble metal catalyst in the FEU, followed by carbon dioxide and water removal in a layer of molecular sieve in the FEU. The temperature of the gas exiting the main air compressor is high enough to effectively oxidize carbon monoxide in the humid feed air, but not hydrogen. Hydrogen is much more difficult to oxidize than carbon monoxide and is oxidized in dry air, where the catalyst activity is typically acceptable even at ambient conditions.
Improved oxidation catalysts have also been described in the literature. For example, U.S. Pat. No. 5,693,302 describes use of a catalyst composed of gold or palladium on a titanium dioxide support for carbon monoxide and hydrogen oxidation. A combination of gold and/or silver with a platinum group metal on a support is proposed in U.S. Pat. No. 5,662,873. Although these catalysts may improve the kinetics of the oxidation reactions, they are inherently expensive.
The above examples conduct at least one oxidation reaction in a separate reactor (upstream of the FEU) operating at elevated temperature.
Earlier processes for the ambient temperature oxidation of carbon monoxide to carbon dioxide in air are given in U.S. Pat. Nos. 3,672,824 and 3,758,666.
U.S. Pat. No. 5,238,670 describes a process for removing carbon monoxide and/or hydrogen from air at a temperature of 0 to 50° C. by (i) removing water from air until it has a water content lower than 150 ppm; and (ii) removing carbon monoxide and hydrogen on a bed of particles containing at least one metallic element selected from copper (Cu), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt), deposited by ion-exchange or impregnation on zeolite, alumina or silica. EP0454531 extends this concept to first remove water and carbon dioxide, then react carbon monoxide and hydrogen at 0 to 50° C. over a zeolite-based catalyst with one metallic element selected from ruthenium (Ru), rhodium (Rh), osmium (Os), iridium (Ir) and platinum (Pt), then distil the purified air stream to produce UHP nitrogen. U.S. Pat. No. 5,478,534 describes use of the same catalyst layer with an upstream adsorbent layer to remove water, together with a downstream layer to remove carbon dioxide and water, before distillation.
U.S. Pat. No. 5,110,569 teaches a process for removing trace quantities of carbon monoxide and hydrogen from an air stream using a three layer adsorption bed having a first layer for adsorbing water (suitably alumina, silica gel, zeolite or combinations thereof), a second layer of catalyst for converting carbon monoxide to carbon dioxide (suitably nickel oxide or a mixture of manganese and copper oxides), and a third layer for adsorbing carbon dioxide and water (suitably zeolite, activated alumina or silica gel). The second layer may include a catalyst for converting hydrogen to water and this may be a supported palladium catalyst. The beds are regenerated by TSA or PSA methods.
In FR2739304, carbon monoxide is first oxidized to carbon dioxide which, together with carbon dioxide and water initially present in the feed air, is adsorbed using conventional adsorbents. Thereafter, hydrogen is adsorbed on a catalyst consisting of palladium (Pd) supported on alumina. The reference indicates that metals that can be used in place of palladium (Pd) are osmium (Os), iridium (Ir), rhodium (Rh), ruthenium (Ru), and platinum (Pt). The reference states that hydrogen is not oxidised under these conditions.
The TSA unit in U.S. Pat. No. 5,202,096 contains a layer of adsorbent(s) to remove water and carbon dioxide, a layer of catalysts to oxidize carbon monoxide and hydrogen, and a layer of adsorbent to adsorb water and carbon dioxide. The purified gas is further processed in a cryogenic process to separate oxygen/nitrogen and to generate a purge gas for the TSA unit. Hopcalite and nickel oxide (NiO) are specified for carbon monoxide oxidation and a precious metal catalyst for hydrogen oxidation. U.S. Pat. No. 6,113,869 applies a similar approach for purification of inert gas (Ar) rather than air where the carbon dioxide removal requirement is omitted.
U.S. Pat. No. 6,048,509 discloses a method and process utilizing a modified precious metal catalyst (platinum or palladium and at least one member selected from the group consisting of iron, cobalt, nickel, manganese, copper, chromium, tin, lead and cerium on alumina) for oxidation of carbon monoxide to carbon dioxide, followed by water removal in an adsorbent layer and carbon dioxide removal in a second adsorbent layer. An option for hydrogen removal is provided with a second catalyst layer (based on palladium/platinum) and adsorbents for water removal in subsequent layers.
U.S. Pat. No. 6,093,379 teaches a process for combined hydrogen and carbon monoxide removal consisting of a first layer to adsorb water and carbon dioxide on alumina or zeolite, and a second layer of a precious metal catalyst (palladium on alumina) to simultaneously oxidize carbon monoxide, adsorb the formed carbon dioxide and chemisorb hydrogen.
Kumar and Deng (2006) and U.S. Pat. No. 6,511,640 teach five layers in the TSA unit; a first layer to remove water, a second layer to oxidize carbon monoxide, a third layer to remove carbon dioxide, a fourth layer to oxidize hydrogen and a final adsorption layer to remove water and carbon dioxide. Hopcalite catalyst is specified for the carbon monoxide oxidation in the lower catalyst layer, and precious metal catalyst must be used to oxidise hydrogen to produce water in the upper catalyst layer. Hydrogen removal occurs by a chemisorption process rather than the typical reaction mechanism, as evidenced by breakthrough curves. Carbon dioxide interferes with the chemisorption and subsequent removal of hydrogen from the gas, so the precious metal catalyst is placed after both water removal (alumina layer) and carbon dioxide removal (13X). A final 13X layer is placed above the precious metal catalyst for capture of any water produced from the hydrogen oxidation.
Therefore, the current art for removing hydrogen, or carbon monoxide and hydrogen, with in-bed technology suffers from a few issues. First, the removal of hydrogen necessitates the use of expensive supported metal catalysts, typically a precious metal catalyst based on palladium, platinum, ruthenium, rhodium and the like, supported on alumina, zeolite, or silica. Precious metals are generally in high demand and subject to market forces, making them very expensive on a unit mass basis. For example, at the time of writing, the catalyst metal cost based on current palladium spot price ($610/oz) and 0.2 wt % palladium loading is around $45/kg catalyst. The metal content cost of a similar platinum catalyst is $115/kg catalyst ($1600/oz Pt spot price). The cost of the support, manufacture of the catalyst, shipping, etc. are additional charges on the final catalyst. In addition, catalysts employing precious metals are often loaded with the minimum amount of metal possible which makes them more susceptible to poisoning.
Secondly, the precious metal catalysts used in the in-bed technology are thermally regenerated in oxygen-containing streams, typically oxygen-enriched waste gas from the ASU. Noble metals are well known to resist oxidation, but over time they will slowly oxidize and lose catalytic activity under these conditions.
Thirdly, in-bed technologies designed to remove both carbon monoxide and hydrogen are often arranged with multiple catalyst layers, one for carbon monoxide oxidation and another for hydrogen oxidation. This arrangement is especially the case when there is reason to conduct the carbon monoxide oxidation and hydrogen oxidation at different locations within the TSA unit. For example, carbon monoxide oxidation is often conducted after water rejection (since water deactivates the catalyst) and before the carbon dioxide rejection (so carbon dioxide formed from the oxidation is removed as well). Hydrogen oxidation is often conducted after both water and carbon dioxide rejection. Precious metal-based catalyst can be specified for the two separate catalyst layers. It is widely known, however, that hopcalite is very effective for converting carbon monoxide to carbon dioxide. It is significantly cheaper than precious metal catalysts. This has led to the widespread specification of hopcalite for the carbon monoxide oxidation catalyst layer and noble metal-based catalyst for the hydrogen oxidation catalyst layer. Alternatively, it is known to use a silver-exchanged zeolite for carbon monoxide adsorption and removal, followed by a metal-based catalyst for hydrogen reaction. Increased layering in a packed bed leads to increased complexity for bed loading and replacement, and additional costs associated with screens for layer segregation. Some TSA vessel designs are not very amenable to multiple bed layers (e.g., radial flow designs), so an excessive number of layers can even make the approach infeasible.
Finally, most TSA designs incorporating oxidation catalyst utilize a final layer of adsorbent to capture any water or carbon dioxide formed from the oxidation of hydrogen and carbon monoxide in the catalyst layers. The adsorbent layer adds volume and therefore cost to the TSA vessel, as well as the cost for the adsorbent. It also adds void volume, which decreases the effective recovery of purified gas from the TSA. Removal of deactivated catalyst is made more difficult, as the adsorbent layer must first be removed. Finally, the catalyst is not as effectively regenerated as it would if it was at the product end of the bed. This is because hot purge gas is passed from the product end to the feed end of the bed, so the impact of heat loss (external losses to the environment and energy used for desorption) becomes more significant as the catalyst layer is placed further from the product end of the bed.
There is a need for new approaches for removing hydrogen, or hydrogen and carbon monoxide, as well as water and carbon dioxide from feed air to a cryogenic distillation process in the production of UHP nitrogen that overcomes at least one, preferably all, of these disadvantages with conventional techniques.